Run hotspot_custom with impl=openj9

[llxia]

serviceability_jvmti_j9 is a test target for openj9. https://github.com/adoptium/aqa-tests/blob/master/openjdk/playlist.xml#L370

When it fails, the Rerun failed hotspot test cases in Grinder with hotspot_custom target link sets TARGET=hotspot_custom and CUSTOM_TARGET=<subtest> (i.e., serviceability/jvmti/vthread/BreakpointInYieldTest/BreakpointInYieldTest.java).

This link could trigger an invalid job as openj9 cannot run with hotspot_custom target. https://github.com/adoptium/aqa-tests/blob/master/openjdk/playlist.xml#L90

To resolve the problem, I think we can allow hotspot_custom to run with openj9 and change the default HOTSPOT_CUSTOM_TARGET to a common subtest. https://github.com/adoptium/aqa-tests/blob/master/openjdk/openjdk.mk#L133